Not applicable.
Not applicable.
1. Field of Invention
This invention relates to a slip base support for tubular posts. Specifically, the invention describes a slip base unit having a casting which includes a triangular, multi-directional base plate and an integral coupler which secures a support post using an internal locking pin.
2. Related Art
Roadside signs can pose a serious safety hazard to motorists. Signs located next to roads pose potential collision points of impact for vehicles. Effective breakaway devices for roadside signs and light pole supports are necessary to achieve the highest levels of highway safety. Therefore, the U.S. Department of Transportation""s Federal Highway Administration (FHWA) policy requires that all roadside sign and light pole supports used on the National Highway System meet the performance criteria contained in the National Cooperative Highway Research Program (NCHRP) Report 350,Recommended Procedures for the Safety Performance Evaluation of H (Report 350). Similarly, State transportation agencies have similar performance criteria for roadside sign and light pole supports. Report 350 was prepared based on research sponsored by the American Association of State Highway and Transportation Officials (AASHTO) in cooperation with the FHWA, and outlines the required criteria for breakaway or yielding supports for signs and luminaries. The three primary appraisal factors for evaluating crash test performance are 1) structural adequacy, 2) occupant risk, and 3) after-collision vehicle trajectory.
Structural adequacy relates to the support""s ability to break away or yield after impact in a predictable manner. The support structure must be able to break away cleanly without undue deformation or any shattering.
Occupant risk relates to the degree of hazard to which occupants in the impacting vehicle would be subjected. Occupant risk is evaluated by the degree of i) detached elements from the support, ii) vehicle rollover, iii) occupant impact velocities, iv) occupant ridedown accelerations, and v) change in vehicle velocity.
Detached elements, fragments or other debris from the sign support structure should not penetrate or show potential for penetrating the occupant compartment of the vehicle, or present an undue hazard to other traffic, pedestrians, or personnel in a work zone. Deformation of, or intrusions into, the occupant compartment that could cause serious injuries should not be permitted. Thus, fragments and components, including connectors, of the sign support system may become dangerous flying projectiles. Units that have unrestrained components, including fasteners and subcomponents, pose a higher number of potential missiles.
Vehicle rollover should not be caused by impact with the sign structure. Systems that have posts that shatter upon impact, creating tire puncture hazards and flipping poles under the vehicle, may pose a rollover hazard.
Occupant impact velocity is the speed at which an unrestrained passenger strikes some part of the vehicle interior such as the instrument panel, window, or door after the vehicle impacts a fixed or moveable object. The maximum allowable occupant impact velocity is 16.40 f.p.s. (5 m/s), with 9.84 f.p.s. (3 m/s) being the preferable maximum. Like occupant ridedown acceleration and change in vehicle velocity, this factor is primarily influenced by the amount of lateral force required to disengage the sign post from its base mounting structure.
Occupant ridedown acceleration is the highest lateral and longitudinal component of resultant vehicular acceleration averaged over any 10-ms interval for the collision pulse subsequent to occupant impact. Occupant ridedown acceleration is a function of the initial change in velocity (acceleration) of the occupant relative to the vehicle immediately after the vehicle impacts a fixed or moveable object. The maximum allowable ridedown acceleration is 20g""s, with 15g""s being the preferred maximum allowable ridedown acceleration.
Change in vehicle velocity is based on the change in velocity of an 1800# (816.5 kg) vehicle immediately after striking a breakaway support at speeds of 20 mph to 60 mph (32 kmph to 97 kmph). The maximum allowable change in velocity is 16 fps (4.87 mps), but preferably does not exceed 10 fps (3.05 mps).
After-collision vehicle trajectory is a measure of the potential of the post-impact trajectory of the vehicle to cause a subsequent multi-vehicle accident. After collision it is preferable that the vehicle""s trajectory not intrude into adjacent traffic lanes. This factor is influenced primarily by the ease with which the sign post breaks away from its base mounting.
To address these and similar safety parameters for crash sign supports, numerous designs have been introduced. Most prior art describes signs that collapse upon impact, but do not xe2x80x9cbreak awayxe2x80x9d. For example, Hugron (U.S. Pat. No. 5,160,111 xe2x80x94Nov. 3, 1992) describes a collapsible signal post having an insert tube connecting a base post and a sign post. The replaceable tubular insert has a helical cut, which allows the top post to bend upon impact. Deficiencies in this design include the non-reusable nature of the tubular insert, due to designed deformation upon impact, making the system expensive to repair/replace. Daggs et al. (U. S. Pat. No. 4,565,466 xe2x80x94Jan. 21, 1986) discloses a spring loaded return jointed sign post pedestal. The sign post mates with a fluted bell, which prevents rotation. Deficiencies include the inability to replace the sign post without replacing the attached base post, since the strength of the spring must be such that field reattachment of the sign and base posts is not practical. Miller (U. S. Pat. No. 2,141,067 xe2x80x94Dec. 20, 1938) utilizes a spring loaded lightweight post. However, this design lacks the ability to support a large sign, due to strength limitations of the spring and its connections.
A commonly used breakaway system is described by Nehls (U.S. Pat. No. 4,926,592 xe2x80x94May 22, 1990). The device has four main components: a ground engaging mounting post, a pedestal mounting member, a support post mounting member, and a support post for the sign. The ground engaging mounting post is buried in the ground, typically embedded in concrete. The pedestal mounting member, with a triangular plate at one end and a shaft at the other, slides its shaft within the ground engaging mounting post, where it is bolted. The support post mounting member also has a triangular plate at one end, and a vertical standard, typically elongated C-channels that form an open sided square cross-section. The triangular plate of the support post mounting member bolts to the triangular plate of the pedestal mounting member, such that there is a bottom plate (connected to the ground support) and a top plate (for connection to the sign post). The support post holding the sign is slid within the C-channels of the support post mounting member, and the post and channels are bolted together. It essential that the C-channels be bolted tightly against the sign post, which has multiple pre-drilled holes for bolt alignment. The triangular plates have notches in their apexes, through which cam bolts are fastened, securing the top plate to the bottom plate. The cam bolts each have a pair of cam rollers around the shaft of the bolt. The first roller is rollable across the interior of a notch of the support post triangular plate/flange, and the second cam is rollable across the interior of a notch if the pedestal mounting triangular plate/flange. When a vehicle strikes the sign post, the top plate slides off the bottom plate, and the cam bolts are ejected laterally out of the notches as the cams rotate. A friction reducing gasket, preferably made of TEFLON, is between the two triangular plates to facilitate the sliding movement of the top plate off the bottom plate. Deficiencies in the Nehls design include the bolting system of the sign post to the post mounting member, the securement of the C-channels to the triangular plate, the friction reducing gasket.
One disadvantage of the Nehls ""592 system relates to the bolting system of the sign post. The system is designed to be used on posts having multiple holes, which facilitate telescoping. Exposed bolts firmly attach the sign post to the C-channels to a tightness level sufficient to eliminate any yield or take up that could occur upon impact, which would add to the breakaway force of the coupling. This connection system poses three problems. First, the exposed bolts are subject to rusting and/or locking up due to environmental exposure. Thus, replacing the sign post within the C-channels is difficult if not infeasible. Second, when the sign post is impacted by a vehicle, the exposed nuts, bolts and washers are free to fly forward, creating projective missile hazards. Third, if mounted transversely to the point of vehicle impact, the bolts form a pivot point about which the sign support may rotate, pressing against the base of the C-channels causing the C-channel welds to the triangular base to fail.
The C-channels are so shaped to afford water drainage away from the sign post. Thus, there must be an open side for the C-shape. Casting such a device is not technologically and economically feasible, thus the C-channels must be welded to the triangular plate. This poses a weak connection, and a source of failure upon vehicular impact. When the weld breaks, the triangular plate and C-channel pieces add to the protective missile debris with the bolts, nuts and washers.
The effectiveness of the friction reducing plate of the Nehls ""592 patent is also limited. Because of the opening in the middle of the TEFLON friction reducing gasket, there is a strong likelihood that upon impact there will be direct metal-to-metal contact between the top triangular plate and the bottom triangular plate. This is typical where the triangular plate rotates upward in the front of the plate due to rotational torque about the sign post upon vehicular impact, causing the back portion of the top plate to drag across the unprotected portion of the bottom plate. Likewise, a crack in the side of the TEFLON gasket can allow the gasket to degrade from water seepage, dramatically reducing its friction reducing capability.
To reduce the amount of force required to break away the coupling, Nehls ""592 requires the use of roller cams around the coupling connector bolts. These cams roll, typically in opposite directions, across the notch surfaces in the upper and lower flanges of the coupler. The present invention does not typically use such cams, and still outperforms the Nehls ""592 design, as shown in Table 1:
Under similar test conditions, the present invention breaks away with much less force than found in the prior art described by Nehls ""592, resulting in less vehicle velocity change after impact, thus resulting in less internal momentum change on occupants.
It would therefore be a useful improvement of the prior art for a sign support slipbase system to smoothly disengage from a ground support upon impact from a vehicle, with a minimum of projective missiles ejected from the disengaged system.
Accordingly, the objectives of this invention are to provide, inter alia, a new and improved breakaway post slipbase that:
easily breaks away from a ground stub base upon impact from a vehicle;
confines connection hardware after impact;
is resistant to harsh environments;
affords reuse of the post; and
is cost efficient.
These objectives are addressed by the structure and use of the inventive breakaway post slipbase. A base stub is embedded in the ground, typically within a concrete footing, which is buried such that the base stub extends approximately 3xe2x80x3 above the ground, terminating at a triangular ground stub base flange. A slip base casting receives a post, typically a thin walled square tubing, such as a sign post. The slip base casting also has a triangular flange, which bolts to the ground base stub flange with flange bolts that pass though corresponding notches in each of the apexes of both triangles. A bolt keeper plate, which is a triangular shaped sheet of thin metal, restricts and aligns the bolts within the notches, maximizing contact area with the flanges, and preventing bolt xe2x80x9ccreepxe2x80x9d over time from vibration and other forces caused by wind and the environment against the supported sign or device. The bolt keeper plate also provides a solid slick surface for a slip plate to slide across during breakaway. This slip plate is secured to the slip base casting by a locking pin, which is inside the slip base casting. The locking pin also has a primary function, which is to secure the sign post to the slip base casting. The locking pin preferably has retaining grommets, to keep the pin in place after impact, thus holding together as one unit the post, slip plate and slip base casting, preventing the pin from becoming a projectile. The design affords smooth breakaway of the coupling between the base stub and slip base casting, such that typically the post, casting, locking pin, grommets and even flange bolts can be reused after breakaway.
Other objects of the invention will become apparent from time to time throughout the specification hereinafter disclosed.